3. How the Plugin Works

This section outlines, in broad strokes, the physical model behind the sound -- just enough to make the controls in Section 2 make sense. It stays high-level on purpose: the exact equations and calibration constants of the shipping plugin are proprietary (see A Note on the Numbers at the end).

The Modelling Approach

The plugin is a physical model: rather than playing back recorded samples, it simulates the chain of physical events that makes a real aeolian harp sing. A single shared wind source blows across 12 strings; each string has its own vortex exciter that turns that wind into an excitation; each excitation drives a digital-waveguide string that produces the tone; the strings feed energy into one another through sympathetic coupling; and the summed strings pass through a body resonance, stereo placement, and a soft-limiting output stage.

Wind (shared)
    |
    v
12x Vortex Exciter --> 12x Waveguide String --> Body + Stereo --> Output
                              ^
                              |
                     Sympathetic coupling
                     (between all 12 strings)

The rest of this section walks that chain, pointing out which control shapes each stage.

The Wind

Real wind gusts and lulls on several timescales at once, and the model layers slow, medium, and fast variation on top of a base wind speed to reproduce that. Wind sets the base speed; Turb scales the fast, moment-to-moment jitter; Gust scales the slow, multi-second swells. The slow swell also governs how hard the wind pushes into the strings, which is what Breath dials in -- during a lull the drive eases off so notes already ringing decay naturally (and sympathetic tails ring on) rather than the whole sound being ducked.

Why Strings Come and Go

As wind flows past a string it sheds vortices at a rate that rises with wind speed and falls with string thickness. A string only sounds when that shedding rate lines up with one of its harmonics -- a narrow "lock-in" window -- which is why the harp's voice shifts continuously as the wind changes, and why Wind and String Length change which strings and harmonics you hear (longer strings have narrower windows and are more selective). That climb does not continue forever: a real string cannot keep taking energy from the wind as the shedding races past its useful register -- the vortex street loses its grip and the losses in the string climb steeply -- so beyond a point the extra wind arrives as broad, buffeting turbulence that pushes on all the string's modes at once instead of singing on one high one. The model does the same, which is why the top of the Wind range reads as more force and more weather rather than more treble. Following wind-tunnel studies of real aeolian harps (Spalding 2021), the model keeps the fundamental almost silent and lets the 3rd harmonic dominate, so the instrument sings in its characteristic upper partials rather than at the played pitch. Unison strings are nudged just enough to lock onto different harmonics, so paired courses spread into a sparse chord instead of a flat unison.

The Strings

Each string is a Karplus-Strong digital waveguide (a technique from Karplus & Strong, 1983, formalised by Julius O. Smith III): a short delay loop tuned to the string's pitch, with a filter in the feedback path. Bright opens or closes that filter -- how much high-frequency content survives each trip around the loop. Damp sets the ring time -- how long the string sustains. String Length sets the loop length -- longer strings ring longer, lock in more selectively, and pull the wind's excitation toward the lower partials so the voice darkens (shorter strings brighten it), making Length a register control as well as a physical dimension. Material (Gut / Metal / Nylon) shifts brightness, decay, and wind responsiveness together, so the three read as three builds of one instrument rather than three different synths. A built-in damping floor means strings always lose energy -- infinite sustain is not possible.

Sympathetic Resonance

On a real harp, a vibrating string sets its neighbours going through the shared bridge and body. The model couples all 12 strings by how many harmonics they share, so strings in simple ratios (unisons, octaves, fifths) trade energy strongly while detuned strings barely couple at all -- and because the coupling tracks the tuning, retuning a string re-derives its coupling live. Sympathy sets how much of this shared energy feeds back into the strings; its top end reaches a stable ceiling, so the coupled strings can never run away into self-oscillation no matter how you tune them.

Body, Stereo, and Output

The summed strings pass through a soundbox resonance that models the hollow wooden body (Body), are spread across the stereo field by constant-power panning (Width, arranged by the Pan Layout selector), and leave through a soft-saturating output stage (Volume) that limits gently at high levels rather than clipping harshly. A gentle fixed tilt takes the last edge off the top octaves on the way out, so the instrument stays easy to listen to at length without dulling its body. When pairing is on, each course's octave partner is blended in by Mix.

A Note on the Numbers

The descriptions above are deliberately qualitative. The specific response curves, filter tunings, lock-in bandwidths, weighting laws, and coupling constants in the shipping plugin are the product of extensive measurement and listening calibration against real aeolian harps, and are proprietary to Daihamu. They are the instrument's lutherie -- the part of the design we keep.